Revisiting Nested Formsets

It’s been nearly four years since I first wrote about nested
formsets. When I wrote about nested formsets, I must have been using
Django 1.1 (based on correlating dates in the release notes and the
original blog post), which means what I wrote has had four major
releases of Django to drift out of date. And yet it’s still one of the
most frequently visited posts on my blog, and one of the few that I
receive email questions about. Four years later, it seemed like the
time to revisit the original post to see if nested formsets still
make sense and if so, what they look like now.

Formsets help manage the complexity of maintaining multiple instances
of a Form on a single page. For example, if you’re editing a list of
items on a single page, each individual item may be a copy of the same
form. Formsets help manage things like HTMLID generation, flagging
forms for deletion, and validating the entire set of forms together.
When used with Models, they allow you to edit the members of a
QuerySet all at once.

So what are nested formsets? The example I used previously was
something along the lines of Block – Building – Tenant: one Block has
many Buildings, and each Building has many Tenants. If you’re editing
a Block, you want to see all the Buildings and all the Tenants at
once. That’s a fine hypothetical, but one of the questions I get with
some frequency is “what’s a good use case for a nested formset?” Four
years later — two and a half of them spent doing web development full
time — I have yet to encounter a situation where I needed a nested
formset. In that time I’ve built some pretty complex forms, including
Eventbrite’s event creation flow. That page was complex
enough that I built Form Groups to support the interaction, and I
think the jury is still out on whether that was a good idea or not.
It’s possible that there are use cases for nested formsets in
admin-style applications that I haven’t encountered. I think it’s also
possible that there are reasons to use a nested formset alongside a
Javascript framework to ease the user experience.

Note that if you only have one level of relationships on the page (ie,
you’re editing all the Tenants for a single Building in our example)
then you don’t need nested formsets: Django’s inline formsets will
work just fine.

And why not nested form sets? From the questions people have asked and
my experience building Form Groups (which borrowed some ideas), I’ve
concluded that they’re difficult to get completely right, have edge
cases that can be hard to manage, and create quite complicated user
interfaces. In my original blog post I alluded to the fact that I
spent most of a three day weekend trying to get the nested formsets to
work right. Two thirds of that time was spent on work I eventually
threw away, because I couldn’t manage the edge cases. It was only when
I started using TDD that I managed to get something working. But I
didn’t publish the tests with my previous code example, so no one else
was able to benefit from that work.

If you’ve read this far and still think a nested formset is the best
solution for your problem, what would that look like with Django 1.5?
The answer is: simpler. I decided to rewrite my initial implementation
using test driven development. The full implementation of the formset
logic only overrides three methods from BaseInlineFormSet.

fromdjango.forms.modelsimport(BaseInlineFormSet,inlineformset_factory,)classBaseNestedFormset(BaseInlineFormSet):defadd_fields(self,form,index):# allow the super class to create the fields as usualsuper(BaseNestedFormset,self).add_fields(form,index)form.nested=self.nested_formset_class(instance=form.instance,data=form.dataifself.is_boundelseNone,prefix='%s-%s'%(form.prefix,self.nested_formset_class.get_default_prefix(),),)defis_valid(self):result=super(BaseNestedFormset,self).is_valid()ifself.is_bound:# look at any nested formsets, as wellforforminself.forms:result=resultandform.nested.is_valid()returnresultdefsave(self,commit=True):result=super(BaseNestedFormset,self).save(commit=commit)forforminself:form.nested.save(commit=commit)returnresult

These three method cover the four areas of functionality I called out
in the previous post: validation (is_valid), saving (both
existing and new objects are handled here by save), and
instantiation (creating the nested formset instances, handled by
add_fields).

By making it a general purpose baseclass, I’m also able to write a
simple factory function, to make using it more in tune with Django’s
built-in model formset.

Of course there’s more needed — templates, for one — but this shows
just how easy it is to create the views and leverage a generic
abstraction. The real keys here are specifying model =
models.Block and the definition of get_form_class. Django’s
UpdateView knows how to implement the basic form processing idiom
(GET, POST, redirect), so all you need to do is tell it which form to use.

You can find a functional, albeit ugly, demo application in the
demo directory of the git repository.

So that’s it: a general purpose, updated implementation of nested
formsets. I advise using them sparingly :).